1. Field of the Invention
The present invention relates to a method of forming metal bumps such as solder bumps or the like and an electronic device having such bumps.
2. Description of the Related Art
In information processing systems for processing vast amounts of information at high speed, LSIs and VLSIs having electronic circuits and electronic parts integrated on a semiconductor chip find wide applications. In order to mount a semiconductor chip having integrated electronic circuits to a ceramic substrate, for example, metal bumps of solder or the like are formed on the semiconductor chip or the ceramic substrate, and the semiconductor chip is mechanically coupled with fixed on and electrically connected to the ceramic substrate, by fusing the solder bumps. For this purpose, it is necessary to form solder bumps on the semiconductor chip or the ceramic substrate or printed circuit board in advance.
Japanese Patent Publications JP-A-7-249631 and JP-A-9-36118 disclose a method of forming solder bumps on an electronic member such as a semiconductor chip, using a bump-forming plate having a plurality of cavities. In this method, a plurality of cavities are formed in the surface of a flat bump-forming plate, the cavities of the bump-forming plate are filled with solder paste by squeegeeing, the bump-forming plate is heated thereby to form the solder balls from the solder paste in the cavities, and the semiconductor chip is moved toward the bump-forming plate thereby to transfer the solder balls from the plate to the semiconductor chip.
As a result of filling the cavities of the bump-forming plate with solder paste by squeegeeing, the surface of the solder paste is flush with the surface of the bump-forming plate, so that a predetermined amount of solder paste is inserted in each of the cavities. The solder component of the solder paste in the cavities of the bump-forming plate is heated and molten, and is rounded by surface tension to form solder balls. Each of the solder balls, while being held in each cavity of the bump-forming plate, has the top thereof protruded upward out of the surface of the bump-forming plate. With the movement of the semiconductor chip toward the bump-forming plate, therefore, electrode pads on the surface of the semiconductor chip come into contact with the tops of the solder balls, with a small gap left between the surface of the semiconductor chip and the surface of the bump-forming plate. In this way, the solder balls can be transferred from the bump-forming plate to the semiconductor chip.
According to this method, the solder bumps are exactly arranged in accordance with the position of the cavities of the bump-forming plate, and solder balls are formed in a uniform size corresponding to the amount of the paste (i.e. the size of each cavity) inserted in the cavities of the plate, thus making it possible to form bumps with high accuracy and low cost. Especially in the case where a bump-forming plate of silicon is used and cavities are formed in the silicon plate by anisotropic etching, a multiplicity of minuscule cavities can be accurately formed at small pitches. This method can thus be suitably used for forming bumps on a semiconductor chip having a high-density wiring at small pitches.
However, the higher the density and hence the smaller the pitches of the wiring of the semiconductor chip, the smaller the solder bumps to be formed, and hence the smaller the extent of the solder ball tops which are protruded from the surface of the bump-forming plate. If a small distortion of the surface of the bump-forming plate or the semiconductor chip exists, a very small proportion of the solder balls are not transferred from the bump-forming plate to the semiconductor chip and may cause a defect.
Also, if the gap between the surface of the semiconductor chip and the surface of the plate becomes smaller at the time of transfer of the solder balls, foreign matters may intrude between the bump-forming plate and the semiconductor chip and may be pressed to the semiconductor chip, with the result that the surface of the semiconductor chip may be damaged or the foreign matters may be undesirably attached to the wiring of the semiconductor chip.
It is therefore desirable to permit the tops of the solder balls to upwardly protrude to a comparatively large measure from the surface of the bump-forming plate, even if the size of the solder balls are reduced.
Further, in operating an electronic device comprising a semiconductor chip and a circuit board coupled to each other by bumps, heat is generated in the electronic circuits of the electronic device and the semiconductor chip and the circuit board are deformed. The difference in coefficient of thermal expansion between the semiconductor chip and the circuit board causes a difference between the amount of deformation of the semiconductor chip and that of the circuit board, thereby causing a stress in the metal bumps coupling the semiconductor chip and the circuit board to each other. Repetitive exertion of stress on the metal bumps progressively accumulates the fatigue of the metal bumps and leads to the problem of a reduced durability or reliability of at least a portion of the metal bumps. The stress exerted on the metal bumps is comparatively low in the central area of the circuit board and comparatively high in the outer peripheral area thereof. The metal bumps located in the outer peripheral area, therefore, tend to be reduced in durability and reliability.